The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Andreas Vieli - One of the best experts on this subject based on the ideXlab platform.
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water controls the seasonal rhythm of rock Glacier Flow
Earth and Planetary Science Letters, 2019Co-Authors: Alessandro Cicoira, Jan Beutel, Jerome Faillettaz, Andreas VieliAbstract:Abstract Rock Glaciers are creeping periglacial landforms experiencing strong acceleration during recent atmospheric warming and raising concerns with regard to their future behaviour and stability. High resolution kinematic observations show strong seasonal and multi-annual variations in rock Glacier creep, but the linking mechanisms to environmental forcing remain poorly understood and lack quantitative models. Here we investigate the interaction between rock Glacier creep and climatic forcing - temperature and precipitation - by developing a novel conceptual and numerical modelling approach. The model is constrained and the results are compared with data from the Dirru Rock Glacier (Vallis - CH). We are able to reproduce the observed velocity variations both in magnitude and phase on seasonal and inter-annual time scales. We find that water from liquid precipitation and snow melt, rather than air temperature, is the main driver of variations in rock Glacier creep. Our results imply that the influence of water on rock Glacier creep is fundamental and must be considered when investigating the historic and future evolution of rock Glaciers.
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multisensor validation of tidewater Glacier Flow fields derived from synthetic aperture radar sar intensity tracking
The Cryosphere, 2019Co-Authors: Christoph Rohner, Jan Beutel, David Small, Daniel Henke, Martin P Luthi, Andreas VieliAbstract:Abstract. Following the general warming trend in Greenland, an increase in calving rates, retreat and ice Flow has been observed at ocean-terminating outlet Glaciers. These changes contribute substantially to the current mass loss of the Greenland Ice Sheet. In order to constrain models of ice dynamics as well as estimates of mass change, detailed knowledge of geometry and ice Flow is needed, in particular on the rapidly changing tongues of ocean-terminating outlet Glaciers. In this study, we validate velocity estimates and spatial patterns close to the calving terminus of such an outlet derived from an iterative offset-tracking method based on synthetic aperture radar (SAR) intensity data with a collection of three independent reference measurements of Glacier Flow. These reference datasets are comprised of measurements from differential GPS, a terrestrial radar interferometer (TRI) and repeated unmanned aerial vehicle (UAV) surveys. Our approach to SAR velocity processing aims at achieving a relatively fine grid spacing and a high temporal resolution in order to best resolve the steep velocity gradients in the terminus area and aims to exploit the 12 d repeat interval of the single-satellite Sentinel-1A sensor. Results from images of the medium-sized ocean-terminating outlet Glacier Eqip Sermia acquired by Sentinel-1A and RADARSAT-2 exhibit a mean difference of 11.5 % when compared to the corresponding GPS measurements. An areal comparison of our SAR velocity fields with independently generated velocity maps from TRI and UAV surveys showed good agreement in magnitude and spatial patterns, with mean differences smaller than 0.7 m d −1 . In comparison with existing operational velocity products, our SAR-derived velocities show an improved spatial velocity pattern near the margins and calving front. There 8 % to 30 % higher surface ice velocities are produced, which has implications on ice fluxes and on mass budget estimates of similarly sized outlet Glaciers. Further, we show that offset tracking from SAR intensity data at relatively low spatio-temporal sampling intervals is a valid method to derive Glacier Flow fields for fast-Flowing Glacier termini of outlet Glaciers and, given the repeat period of 12 d of the Sentinel-1A sensor (6 d with Sentinel-1B), has the potential to be applied operationally in a quasi-continuous mode.
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resolving the influence of temperature forcing through heat conduction on rock Glacier dynamics a numerical modelling approach
The Cryosphere, 2019Co-Authors: Alessandro Cicoira, Jan Beutel, Jerome Faillettaz, Isabelle Gartnerroer, Andreas VieliAbstract:Abstract. In recent years, observations have highlighted seasonal and interannual variability in rock Glacier Flow. Temperature forcing, through heat conduction, has been proposed as one of the key processes to explain these variations in kinematics. However, this mechanism has not yet been quantitatively assessed against real-world data. We present a 1-D numerical modelling approach that couples heat conduction to an empirically derived creep model for ice-rich frozen soils. We use this model to investigate the effect of thermal heat conduction on seasonal and interannual variability in rock Glacier Flow velocity. We compare the model results with borehole temperature data and surface velocity measurements from the PERMOS and PermaSense monitoring network available for the Swiss Alps. We further conduct a model sensitivity analysis in order to resolve the importance of the different model parameters. Using the prescribed empirically derived rheology and observed near-surface temperatures, we are able to model the correct order of magnitude of creep. However, both interannual and seasonal variability are underestimated by an order of magnitude, implying that heat conduction alone cannot explain the observed variations. Therefore, we conclude that non-conductive processes, likely linked to water availability, must dominate the short-term velocity signal.
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multisensor validation of tidewater Glacier Flow fields derived fromsar intensity tracking
The Cryosphere Discussions, 2019Co-Authors: Christoph Rohner, David Small, Daniel Henke, Martin P Luthi, Andreas VieliAbstract:Abstract. Following the general warming trend in Greenland, an increase in calving rates, retreat and ice Flow has been observed at ocean-terminating outlet Glaciers. These changes contribute substantially to the current mass loss of the Greenland Ice Sheet. In order to constrain models of ice dynamics as well as estimates of mass change, detailed knowledge of geometry and ice-Flow are needed, in particular on the rapidly changing tongues of ocean-terminating outlet Glaciers. In this study, we validate velocity estimates and spatial patterns close to the calving terminus of such an outlet derived from an iterative offset tracking method based on SAR intensity data with a collection of three independent reference measurements of Glacier Flow. These reference data sets are comprised of measurements from differential GPS, a Terrestrial Radar Interferometer (TRI) and repeated UAV surveys. Our approach for the SAR-velocity processing aims achieving at high spatial and temporal resolution in order to best resolve the steep velocity gradients in the terminus area and to exploit the 12 day repeat interval of the single-satellite Sentinel-1A sensor. Results from images of the medium-sized ocean terminating outlet Glacier Eqip Sermia acquired by Sentinel-1A and RADARSAT-2 exhibit a mean difference of 8.7 % when compared to the corresponding GPS measurements. An areal comparison of our SAR velocity-fields with independently generated velocity maps from TRI and UAV showed a good agreement in magnitude and spatial patterns, with mean differences smaller than 0.7 md−1. In comparison with existing operational velocity products, our SAR-derived velocities showed a strongly improved spatial velocity pattern near the margins and calving front. There 10 % to 20 % higher surface ice velocities are produced, which has substantial implications on ice fluxes and on mass budget estimates of ice sheets. Further, we showed that offset tracking from SAR intensity data at a high spatio-temporal resolution is a valid method to derive Glacier Flow fields for fast-Flowing Glacier termini of outlet Glaciers and, given the repeat interval of 12 days of the Sentinel-1A sensor (6 days with Sentinel-1B), has the potential to be applied operationally in a quasi-continuous mode.
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future sea level rise from greenland s main outlet Glaciers in a warming climate
Nature, 2013Co-Authors: Andreas Vieli, Ian Joughin, M L Andersen, F M Nick, Antony J Payne, Tamsin L Edwards, Frank PattynAbstract:Over the past decade, ice loss from the Greenland Ice Sheet increased as a result of both increased surface melting and ice discharge to the ocean. The latter is controlled by the acceleration of ice Flow and subsequent thinning of fast-Flowing marine-terminating outlet Glaciers. Quantifying the future dynamic contribution of such Glaciers to sea-level rise (SLR) remains a major challenge because outlet Glacier dynamics are poorly understood. Here we present a Glacier Flow model that includes a fully dynamic treatment of marine termini. We use this model to simulate behaviour of four major marine-terminating outlet Glaciers, which collectively drain about 22 per cent of the Greenland Ice Sheet. Using atmospheric and oceanic forcing from a mid-range future warming scenario that predicts warming by 2.8 degrees Celsius by 2100, we project a contribution of 19 to 30 millimetres to SLR from these Glaciers by 2200. This contribution is largely (80 per cent) dynamic in origin and is caused by several episodic retreats past overdeepenings in outlet Glacier troughs. After initial increases, however, dynamic losses from these four outlets remain relatively constant and contribute to SLR individually at rates of about 0.01 to 0.06 millimetres per year. These rates correspond to ice fluxes that are less than twice those of the late 1990s, well below previous upper bounds. For a more extreme future warming scenario (warming by 4.5 degrees Celsius by 2100), the projected losses increase by more than 50 per cent, producing a cumulative SLR of 29 to 49 millimetres by 2200.
Alun Hubbard - One of the best experts on this subject based on the ideXlab platform.
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physical conditions of fast Glacier Flow 1 measurements from boreholes drilled to the bed of store Glacier west greenland
Journal of Geophysical Research, 2018Co-Authors: S H Doyle, Bryn Hubbard, Poul Christoffersen, Tun Jan Young, Coen Hofstede, Marion Bougamont, Jason E Box, Alun HubbardAbstract:Marine-terminating outlet Glaciers of the Greenland ice sheet make significant contributions to global sea level rise, yet the conditions that facilitate their fast Flow remain poorly constrained owing to a paucity of data. We drilled and instrumented seven boreholes on Store Glacier, Greenland, to monitor subglacial water pressure, temperature, electrical conductivity and turbidity along with englacial ice temperature and deformation. These observations were supplemented by surface velocity and meteorological measurements to gain insight into the conditions and mechanisms of fast Glacier Flow. Located 30km from the calving front, each borehole drained rapidly on attaining ∼600m depth indicating a direct connection with an active subglacial hydrological system. Persistently high subglacial water pressures indicate low effective pressure (180 − 280 kPa), with small amplitude variations correlated with notable peaks in surface velocity driven by the diurnal melt cycle and longer periods of melt and rainfall. The englacial deformation profile determined from borehole tilt measurements indicates that 63-71% of total ice motion occurred at the bed, with the remaining 29-37% predominantly attributed to enhanced deformation in the lowermost 50-100 m of the ice column. We interpret this lowermost 100m to be formed of warmer, pre-Holocene ice overlying a thin (0 − 8 m) layer of temperate basal ice. Our observations are consistent with a spatially-extensive and persistently-inefficient subglacial drainage system that we hypothesize comprises drainage both at the ice-sediment interface and through subglacial sediments. This configuration has similarities to that interpreted beneath dynamically-analogous Antarctic ice streams, Alaskan tidewater Glaciers, and Glaciers in surge.
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the response of petermann Glacier greenland to large calving events and its future stability in the context of atmospheric and oceanic warming
Journal of Glaciology, 2012Co-Authors: F M Nick, Andreas Vieli, Alun Hubbard, Adrian Luckman, C J Van Der Veen, D Van As, R S W Van De Wal, Frank Pattyn, D FloricioiuAbstract:This study assesses the impact of a large 2010 calving event on the current and future stability of Petermann Glacier, Greenland, and ascertains the Glacier's interaction with different components of the climate and ocean system. We use a numerical ice-Flow model that captures the major aspects of the Glacier's mass budget, the resistive forces controlling Glacier Flow, and includes dynamic calving. Satellite observations and model results show that the recent break-off of 25% of the floating tongue did not result in a significant Glacier speed-up due to the low lateral resistance of this relatively wide and thin ice tongue. We demonstrate that seasonal speed-up at Petermann Glacier is mainly driven by meltwater lubrication rather than freeze-up conditions in the fjord. Results also show that sub-shelf ocean melt may have a profound effect on the future stability of Petermann Glacier, emphasizing the urgent need for more observations, and a better understanding of fjord temperature variability and circulation.
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comparison of a three dimensional model for Glacier Flow with field data from haut Glacier d arolla switzerland
Journal of Glaciology, 1998Co-Authors: Alun Hubbard, Heinz Blatter, Peter Nienow, Douglas Mair, Bryn HubbardAbstract:A three-dimensional, finite-difference model based on a first-order solution of the ice-Flow equations is applied to Haut Glacier d’Arolla, Switzerland. The numerical model successfully converges at horizontal resolutions down to 70 m, so a number of detailed comparisons with field data can be made. Modelled surface velocities with no basal sliding component are compared with surface velocities observed on the Glacier over four different time periods. The best fit is achieved with over-winter surface velocities ( R 2 = 0.75) using a rate factor, A, in Glen’s Flow law of 0.063 a −1 bar −3 . Surface zones of maximum computed effective stress display a high level of coincidence with observed crevassing, the orientation of which is successfully predicted by the direction of the tensile component of the computed principal surface stress. Comparison of the relative magnitude and direction of computed principal stresses with principal strains measured at the ice surface also correspond closely. In an attempt to simulate the observed annual velocity distribution within a cross-section of the Glacier tongue, we incorporate two basal-motion patterns into the model. By treating net annual ice motion as a time-weighted composite of three separate Flow situations: normal sliding, enhanced sliding and no sliding, we are able to reproduce the key features of the observed cross-sectional ice and basal slip velocity distributions. These experiments indicate there may be substantial decoupling taking place along an elongated narrow zone at the bed of Haut Glacier d’Arolla and that this decoupling interacts in a complex manner with the englacial stress and strain field.
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comparison of a three dimensional model for Glacier Flow with field data from haut Glacier d arolla switzerland
Journal of Glaciology, 1998Co-Authors: Alun Hubbard, Heinz Blatter, Peter Nienow, Douglas Mair, Bryn HubbardAbstract:A three-dimensional, finite-difference model based on a first-order solution of the ice-Flow equations is applied to Haut Glacier d'Arolla, Switzerland. The numerical model successfully converges at horizontal resolutions down to 70 m, so a number of detailed comparisons with field data can be made. Modelled surface velocitics with no basal sliding component are compared with surface velocities observed on the Glacier over four different time periods. The best fit is achieved with over-winter surface velocities (R 2 = 0.75) using a rate factor, A. in Glen's Flow law of 0.063 a -1 bar -3 . Surface zones of maximum computed effective stress display a high level of coincidence with observed crevassing, the orientation of which is successfully predicted by the direction of the tensile component of the computed principal surface stress. Comparison of the relative magnitude and direction of computed principal stresses with principal strains measured at the ice surface also correspond closely. In an attempt to simulate the observed annual velocity distribution within a cross-section of the Glacier tongue, we incorporate two basal-motion patterns into the model. By treating net annual ice motion as a time-weighted composite of three separate Flow situations: normal sliding, enhanced sliding and no sliding, we are able to reproduce the key features of the observed cross-sectional ice and basal slip velocity distributions. These experiments indicate there may be substantial decoupling taking place along an elongated narrow zone at the bed of Haut Glacier d'Arolla and that this decoupling interacts in a complex manner with the englacial stress and strain field.
Christian Schoof - One of the best experts on this subject based on the ideXlab platform.
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Thin-Film Flows with Wall Slip: An Asymptotic Analysis of Higher Order Glacier Flow Models
Quarterly Journal of Mechanics and Applied Mathematics, 2010Co-Authors: Christian Schoof, Richard C. A. HindmarshAbstract:Free-surface thin-film Flows can principally be described by two types of models. Lubrication models assume that shear stresses are dominant in the force balance of the Flow and are appropriate where there is little or no slip at the base of the Flow. Conversely, membrane or 'free-film' models are appropriate in situations where there is rapid slip and normal (or extensional) stresses play a significant role in force balance. In some physical applications, notably in glaciology, both rapid and slow slip can occur within the same fluid film. In order to capture the dynamics of rapid and slow slip in a single model that describes the entire fluid film, a hybrid of membrane and lubrication models is therefore required. Several of these hybrid models have been constructed on an ad hoc basis in glaciology, where they are usually termed 'higher order models'. Here, we present a self-consistent asymptotic analysis of the most common of these models due originally to Blatter. We show that Blatter's model reproduces the solution to the underlying Stokes equations to second order in the film's aspect ratio, regardless of the amount of slip at the base of the fluid. In doing so, we also construct asymptotic expansions for the Stokes equations to this order for shear-thinning power-law fluids, paying particular attention to a high-viscosity boundary layer that develops at the free surface when there is little or no slip at the base. Lastly, we demonstrate that a depth-integrated hybrid model of comparable accuracy to Blatter's model-which cannot be depth integrated-can also be constructed, which we suggest as a viable tool for numerical simulations of thin films that contain both slowly and rapidly sliding parts.
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coulomb friction and other sliding laws in a higher order Glacier Flow model
Mathematical Models and Methods in Applied Sciences, 2010Co-Authors: Christian SchoofAbstract:We consider a widely used higher-order Glacier Flow model with a variety of parametrizations of wall slip, including Coulomb friction, regularized Coulomb friction laws and a power law. Mathematically, the Coulomb friction problem is found to be analogous to a classical friction problem in elasticity theory. We specifically analyze the case in which slip is possible everywhere at the boundary, in which case the weak formulation becomes a semi-coercive convex minimization problem which has a solution only if a solvability condition representing force and torque balance is satisfied. Going beyond previous work, we study the uniqueness of solutions in depth, finding that non-unique solutions are possible under very specialized circumstances. Further, in an extension of work by Campos, Oden and Kikuchi, we show that solutions to the regularized Coulomb friction and power law problems converge to the Coulomb friction problem in appropriate parametric limits, provided the latter is unique, and briefly discuss the implications of possible non-unique solutions for a priori error estimation in numerical approximations.
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variational methods for Glacier Flow over plastic till
Journal of Fluid Mechanics, 2006Co-Authors: Christian SchoofAbstract:We investigate the mechanics of ice streams and Glaciers Flowing over a bed consisting of Coulomb-plastic subglacial sediment, or more generally, of channel Flows with Coulomb or ‘solid’ friction laws at the boundary. Sliding is assumed to occur if shear stress at the Glacier bed attains a prescribed, locally defined yield stress, while no sliding is assumed possible below that yield stress. Importantly, the location of regions of slip and no slip at the bed is not known a priori, but forms part of the solution. By analogy with friction problems in elasticity, we derive a weak formulation as a semi-coercive variational inequality, which admits a unique solution provided a solvability condition ensuring force balance is satisfied. The variational formulation is then exploited to calculate numerical solutions, and we investigate the effect of variations in subglacial water pressure, ice thickness and surface slope on the discharge of a valley Glacier with a plastic bed. Significant differences are found between the behaviour of wide and narrow as well as steep and shallow-angled Glaciers, and our results further indicate the need to develop models capable of accounting for longitudinal stresses.
Jan Beutel - One of the best experts on this subject based on the ideXlab platform.
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water controls the seasonal rhythm of rock Glacier Flow
Earth and Planetary Science Letters, 2019Co-Authors: Alessandro Cicoira, Jan Beutel, Jerome Faillettaz, Andreas VieliAbstract:Abstract Rock Glaciers are creeping periglacial landforms experiencing strong acceleration during recent atmospheric warming and raising concerns with regard to their future behaviour and stability. High resolution kinematic observations show strong seasonal and multi-annual variations in rock Glacier creep, but the linking mechanisms to environmental forcing remain poorly understood and lack quantitative models. Here we investigate the interaction between rock Glacier creep and climatic forcing - temperature and precipitation - by developing a novel conceptual and numerical modelling approach. The model is constrained and the results are compared with data from the Dirru Rock Glacier (Vallis - CH). We are able to reproduce the observed velocity variations both in magnitude and phase on seasonal and inter-annual time scales. We find that water from liquid precipitation and snow melt, rather than air temperature, is the main driver of variations in rock Glacier creep. Our results imply that the influence of water on rock Glacier creep is fundamental and must be considered when investigating the historic and future evolution of rock Glaciers.
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multisensor validation of tidewater Glacier Flow fields derived from synthetic aperture radar sar intensity tracking
The Cryosphere, 2019Co-Authors: Christoph Rohner, Jan Beutel, David Small, Daniel Henke, Martin P Luthi, Andreas VieliAbstract:Abstract. Following the general warming trend in Greenland, an increase in calving rates, retreat and ice Flow has been observed at ocean-terminating outlet Glaciers. These changes contribute substantially to the current mass loss of the Greenland Ice Sheet. In order to constrain models of ice dynamics as well as estimates of mass change, detailed knowledge of geometry and ice Flow is needed, in particular on the rapidly changing tongues of ocean-terminating outlet Glaciers. In this study, we validate velocity estimates and spatial patterns close to the calving terminus of such an outlet derived from an iterative offset-tracking method based on synthetic aperture radar (SAR) intensity data with a collection of three independent reference measurements of Glacier Flow. These reference datasets are comprised of measurements from differential GPS, a terrestrial radar interferometer (TRI) and repeated unmanned aerial vehicle (UAV) surveys. Our approach to SAR velocity processing aims at achieving a relatively fine grid spacing and a high temporal resolution in order to best resolve the steep velocity gradients in the terminus area and aims to exploit the 12 d repeat interval of the single-satellite Sentinel-1A sensor. Results from images of the medium-sized ocean-terminating outlet Glacier Eqip Sermia acquired by Sentinel-1A and RADARSAT-2 exhibit a mean difference of 11.5 % when compared to the corresponding GPS measurements. An areal comparison of our SAR velocity fields with independently generated velocity maps from TRI and UAV surveys showed good agreement in magnitude and spatial patterns, with mean differences smaller than 0.7 m d −1 . In comparison with existing operational velocity products, our SAR-derived velocities show an improved spatial velocity pattern near the margins and calving front. There 8 % to 30 % higher surface ice velocities are produced, which has implications on ice fluxes and on mass budget estimates of similarly sized outlet Glaciers. Further, we show that offset tracking from SAR intensity data at relatively low spatio-temporal sampling intervals is a valid method to derive Glacier Flow fields for fast-Flowing Glacier termini of outlet Glaciers and, given the repeat period of 12 d of the Sentinel-1A sensor (6 d with Sentinel-1B), has the potential to be applied operationally in a quasi-continuous mode.
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resolving the influence of temperature forcing through heat conduction on rock Glacier dynamics a numerical modelling approach
The Cryosphere, 2019Co-Authors: Alessandro Cicoira, Jan Beutel, Jerome Faillettaz, Isabelle Gartnerroer, Andreas VieliAbstract:Abstract. In recent years, observations have highlighted seasonal and interannual variability in rock Glacier Flow. Temperature forcing, through heat conduction, has been proposed as one of the key processes to explain these variations in kinematics. However, this mechanism has not yet been quantitatively assessed against real-world data. We present a 1-D numerical modelling approach that couples heat conduction to an empirically derived creep model for ice-rich frozen soils. We use this model to investigate the effect of thermal heat conduction on seasonal and interannual variability in rock Glacier Flow velocity. We compare the model results with borehole temperature data and surface velocity measurements from the PERMOS and PermaSense monitoring network available for the Swiss Alps. We further conduct a model sensitivity analysis in order to resolve the importance of the different model parameters. Using the prescribed empirically derived rheology and observed near-surface temperatures, we are able to model the correct order of magnitude of creep. However, both interannual and seasonal variability are underestimated by an order of magnitude, implying that heat conduction alone cannot explain the observed variations. Therefore, we conclude that non-conductive processes, likely linked to water availability, must dominate the short-term velocity signal.
Andreas Kaab - One of the best experts on this subject based on the ideXlab platform.
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weekly Glacier Flow estimation from dense satellite time series using adapted optical Flow technology
Frontiers in Earth Science, 2017Co-Authors: Bas Altena, Andreas KaabAbstract:Contemporary optical remote sensing satellites or constellations of satellites can acquire imagery at sub-weekly or even daily timescales. Thus, these systems facilitate the potential for within-season velocity estimation of Glacier surfaces. State-of-the-art techniques for displacement estimation are based on matching image pairs and are thus constrained by the need of significant displacement and/or preservation of the surface over time. Consequently, such approaches cannot benefit entirely from the increasing satellite revisit times. Here, we explore an approach that is fundamentally different from image correlation or similar techniques and exploits the concept of optical Flow. Our goal is to assess if this concept could overcome above current limitations of image matching and thus give new insights in Glacier Flow dynamics. We implement two different methods of optical Flow, and test these on the SPOT5 Take5 dataset over Kronebreen, Svalbard and over Kaskawulsh Glacier, Yukon. For Kaskawulsh Glacier we are able to extract seasonal velocity variation, that temporally coincide with events of increased air temperatures. Furthermore, even for the cloudy dataset of Kronebreen, we were able to extract spatio-temporal trajectories which correlate well with measured GPS Flow paths. Because the underlying concept is simple and computationally efficient due to data-reduction, our methodology can easily be used for exploratory regional studies of several Glaciers or estimation of small and slow Flowing Glaciers.
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Glacier remote sensing using sentinel 2 part i radiometric and geometric performance and application to ice velocity
Remote Sensing, 2016Co-Authors: Andreas Kaab, Solveig H Winsvold, Bas Altena, Christopher Nuth, Thomas Nagler, Jan WuiteAbstract:With its temporal resolution of 10 days (five days with two satellites, and significantly more at high latitudes), its swath width of 290 km, and its 10 m and 20 m spatial resolution bands from the visible to the shortwave infrared, the European Sentinel-2 satellites have significant potential for Glacier remote sensing, in particular mapping of Glacier outlines and facies, and velocity measurements. Testing Level 1C commissioning and ramp-up phase data for initial sensor quality experiences, we find a high radiometric performance, but with slight striping effects under certain conditions. Through co-registration of repeat Sentinal-2 data we also find lateral offset patterns and noise on the order of a few metres. Neither of these issues will complicate most typical glaciological applications. Absolute geo-location of the data investigated was on the order of one pixel at the time of writing. The most severe geometric problem stems from vertical errors of the DEM used for ortho-rectifying Sentinel-2 data. These errors propagate into locally varying lateral offsets in the images, up to several pixels with respect to other georeferenced data, or between Sentinel-2 data from different orbits. Finally, we characterize the potential and limitations of tracking Glacier Flow from repeat Sentinel-2 data using a set of typical Glaciers in different environments: Aletsch Glacier, Swiss Alps; Fox Glacier, New Zealand; Jakobshavn Isbree, Greenland; Antarctic Peninsula at the Larsen C ice shelf.
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accuracy assessment for mapping Glacier Flow velocity and detecting Flow dynamics from aster satellite imagery tasman Glacier new zealand
Remote Sensing of Environment, 2013Co-Authors: Todd A N Redpath, Pascal Sirguey, Sean J Fitzsimons, Andreas KaabAbstract:Abstract Image matching via correlation of remotely sensed imagery is an increasingly popular technique for measuring Glacier Flow because it can provide spatially distributed measurements while mitigating many of the challenges accompanying in situ measurements of Glacier velocity. Despite the increasing use of such techniques, and the requirement for well understood uncertainties where analysis of temporal variability is sought, studies that incorporate an assessment of accuracy based on in situ data remain rare. This study presents a rigorous methodology to quantify the uncertainties that apply to Glacier Flow velocities derived from optical satellite imagery. This method is applied on three successive ASTER images of Tasman Glacier, New Zealand. Evaluation of the quality of co-registration between image pair constituents allows unique uncertainties to be calculated for individual velocity measurements. These uncertainties have magnitudes that correspond to sub-pixel co-registration errors and account for anisotropic co-registration variance. Compared to estimated velocities, uncertainties on the order of 2–30% are achieved. Flow-fields for the 2009–2010 and 2010–2011 periods were derived using various spectral bands and validated with in situ data obtained by GPS survey over a nearly coincident period. This revealed the importance of considering the spectral characteristics of the target surface when selecting bands for image correlation, while validation demonstrated the success of the implemented methodology. Uncertainties derived for the Flow-fields permitted statistical significance of velocity change between measurement periods to be assessed. Significant velocity changes, observed across the Glacier surface, suggest that the Tasman Glacier behaves in a more dynamic way than previously recognised, highlighting the advantages of the methodology described here.
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measurement of surface displacement and deformation of mass movements using least squares matching of repeat high resolution satellite and aerial images
Remote Sensing, 2012Co-Authors: Misganu Debellagilo, Andreas KaabAbstract:Displacement and deformation are fundamental measures of Earth surface mass movements such as Glacier Flow, rockGlacier creep and rockslides. Ground-based methods of monitoring such mass movements can be costly, time consuming and limited in spatial and temporal coverage. Remote sensing techniques, here matching of repeat optical images, are increasingly used to obtain displacement and deformation fields. Strain rates are usually computed in a post-processing step based on the gradients of the measured velocity field. This study explores the potential of automatically and directly computing velocity, rotation and strain rates on Earth surface mass movements simultaneously from the matching positions and the parameters of the geometric transformation models using the least squares matching (LSM) approach. The procedures are exemplified using bi-temporal high resolution satellite and aerial images of Glacier Flow, rockGlacier creep and land sliding. The results show that LSM matches the images and computes longitudinal strain rates, transverse strain rates and shear strain rates reliably with mean absolute deviations in the order of 10−4 (one level of significance below the measured values) as evaluated on stable grounds. The LSM also improves the accuracy of displacement estimation of the pixel-precision normalized cross-correlation by over 90% under ideal (simulated) circumstances and by about 25% for real multi-temporal images of mass movements.
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sub pixel precision image matching for measuring surface displacements on mass movements using normalized cross correlation
Remote Sensing of Environment, 2011Co-Authors: Misganu Debellagilo, Andreas KaabAbstract:This study evaluates the performance of two fundamentally different approaches to achieve sub-pixel precision of normalised cross-correlation when measuring surface displacements on mass movements from repeat optical images. In the first approach, image intensities are interpolated to a desired sub-pixel resolution using a bi-cubic interpolation scheme prior to the actual displacement matching. In the second approach, the image pairs are correlated at the original image resolution and the peaks of the correlation coefficient surface are then located at the desired sub-pixel resolution using three techniques, namely bi-cubic interpolation, parabola fitting and Gaussian fitting. Both principal approaches are applied to three typical mass movement types: rockGlacier creep, Glacier Flow and land sliding. In addition, the influence of pixel resolution on the accuracies of displacement measurement using image matching is evaluated using repeat images resampled to different spatial resolutions. Our results show that bi-cubic interpolation of image intensity performs best followed by bi-cubic interpolation of the correlation surface. Both Gaussian and parabolic peak locating turn out less accurate. By increasing the spatial resolution (i.e. reducing the ground pixel size) of the matched images by 2 to 16 times using intensity interpolation, 40% to 80% reduction in mean error in reference to the same resolution original image could be achieved. The study also quantifies how the mean error, the random error, the proportion of mismatches and the proportion of undetected movements increase with increasing pixel size (i.e. decreasing spatial resolution) for all of the three mass movement examples investigated.